Achievable Rate Maximization in Orbital Angular Momentum-Based Downlink Non-Orthogonal Multiple Access Systems

Orbital angular momentum (OAM)-based communication system is promising to increase the spectrum efficiency by using orthogonal OAM-modes to transmit different signals simultaneously. To further increase the spectrum efficiency, incorporating the non-orthogonal multiple access (NOMA) scheme with the OAM-based communication system is proposed. For an OAM-based downlink NOMA system which consists of a base station, a near user, and a far user, the achievable rate maximization problem is studied, which maximizes the achievable rate of the near user under the minimum transmission rate constraint at the far user. To solve the non-convex problem, a majorization-minimization based iterative algorithm is proposed, where in each iteration, a low-complexity Lagrange dual method is employed. It is shown from simulation results that the proposed NOMA system is superior than the conventional orthogonal multiple access system.